1. Field of the Invention
The present invention relates to a method of detecting dust and a method of preventing erroneous determination of dust detection during resistance welding such as spot welding.
2. Related Art
According to the related art, a lot of welding robots which spot-weld the predetermined part of a work conveyed by a conveyance device are arranged in a production line for welding car bodies and assembling thereof. These welding robots are provided with a spot welding gun having a shape appropriate for the predetermined welding part of a work.
These kind of welding robots monitor a resistance value while a welding current is applied. When dust is generated while a welding current is applied, a considerably decreased resistance value is monitored.
In the case in which such dust is generated, the strength of the welding part may be insufficient even after a predetermined welding current is applied for a predetermined current applying time. This is because the growing rate of a nugget decreases by the generated dust, and the size of the nugget becomes small.
Thus, welding robots are programmed so as to determine that dust is generated and perform a predetermined process on dust detection to shut off, increase, or decrease a welding current when the resistance value decreases while a welding current is applied (refer to Unexamined Japanese Patent Application, First Publication No. 2006-55893).
By the way, when welding operation is performed, for example, under condition in which the spot welding guns of two adjacent welding robots are close to each other, the magnetic field of the first spot welding gun affects that of the second spot welding gun, so that the resistance value may temporarily decrease. Such decrease of the resistance value is temporal, which is a noise fading out at once, not affecting welding quality.
However, on such noise generation, welding robots may erroneously determine that dust is generated due to the decrease of the resistance value. When erroneously determining that dust is generated in this way, welding robots perform a predetermined process on dust detection to shut off, increase, or decrease a welding current.
The process on dust detection based on such erroneous determination is performed, causing improper operation such as uselessly stopping welding operation and setting a welding current to an abnormal value.
In addition, when foreign matter such as dust enters between material members to be welded, the initial resistance value for resistance welding extremely increases compared with that at the start of normal resistance welding. Then, the foreign matter melts, so that the initial resistance value may rapidly decrease. In this case, welding robots erroneously determine that dust is generated due to the rapid decrease of the initial resistance value and perform a predetermined process on dust detection to shut off, increase, or decrease a welding current.
The process on dust detection based on such erroneous determination is performed, causing improper operation such us uselessly stopping welding operation and setting a welding current to an abnormal value.